1. Field of the Invention
This invention relates to a paper for electrostatography using an encapsulated toner, and more particularly to a paper for receiving on its surface a visible image of an encapsulated toner, such as a paper employable in copying machine, printing machine or line printer utilizing electrostatography.
2. Description of Prior Arts
There is known an electrostatography which comprises a stage of developing a tone electrostatic latent image contained on a photoconductive or dielectric surface with a toner material containing a colorant and a fixing aid (i.e., binder) to produce a visible toner image, and a subsequent stage of transferring and fixing the visible toner image onto a surface of a substrate (i.e., support medium) such as a paper.
The development of the latent image to produce a visible toner image is carried out by the use of either a developing agent consisting essentially of a combination of a toner with carrier particles, or a developing agent consisting essentially of a toner only. The developing process utilizing the combination of a toner with carrier particles is named "two component developing process", while the developing process utilizing only a toner is named "one component developing process".
The toner image formed on the latent image is then transferred onto a surface of a substrate and fixed thereto. The process for fixing the toner image to the substrate can be done through one of three fixing processes, that is, a heat fixing process (fusion process), a solvent fixing process and a pressure fixing process.
The pressure fixing process which involves fixing the toner onto the surface of a substrate under application of pressure thereto is described, for instance, in U.S. Pat. No. 3,269,526. The pressure fixing process involving the use of neither a heating procedure nor a solvent produces no such troubles as inherently attached to either the heat fixing process or the solvent fixing process. Moreover, the pressure fixing process can be employed in conjunction with a high speed automatic copying and duplicating process, and the access time is very short in the pressure fixing process. Accordingly, the pressure fixing process is considered to be an advantageous fixing process inherently having a variety of preferable features.
However, the pressure fixing process also has certain inadvantageous features. For instance, the pressure fixing process generally shows poorer toner fixing property than the heat fixing process does, whereby the toner image fixed onto a paper is apt to rub off easily. Further, the pressure fixing process requires very high pressure for performing the fixing operation, and such high pressure tends to break the cellulose fibers of the substrate such as paper and also produces unsatisfactorily glossy surface on the substrate. Moreover, the pressing roller requires to have relatively greater size, because the roller necessarily imparts very high pressure to the toner image placed on the substrate. Accordingly, a reduction of the size of a copying machine cannot exceed a certain limit defined by the size of a pressing roller.
There has been previously proposed an encapsulated toner which comprises toner particles enclosed with microcapsules, so as to overcome the above-described disadvantageous features of the pressure fixing process. The encapsulated toner is generally prepared by enclosing a core material (containing a colorant such as carbon black) with a shell which is rupturable by the application of pressure in the developing stage. Thus prepared encapsulated toner has various advantageous features; for instance, fixing of the encapsulated toner does not require very high pressure but the toner fixing property is high. Accordingly, the encapsulated toner is viewed as suitable for the use in the pressure fixing process. However, a copying operation using encapsulated toners proposed up to now appear unsatisfactory in giving a visible toner image of high quality. This unsatisfactory result arises from some unsatisfactory characteristics of the toner as well as from certain unfitness of a paper substrate employed.
A paper employed as a substrate for receiving the visible image of encapsulated toner is required to show the following characteristics: the toner image transferred on the paper can be easily and firmly fixed thereonto; the paper is substantially free from troubles possibly occurring in the paper supply system of the coyping machine, such as paper clogging, double supply, misfeeding, and troubles in the course of introducing the paper into a tray or sorter; the paper retains the fixed toner image with high quality; and the paper is satisfactory in the appearance, for instance, with respect to curling, hue, crease, and dimensional accuracy. However, the papers heretofore employed in the electrostatography using encapsulated toners are not satisfactory in certain features of the above-mentioned characteristics.